The glycoprotein hormones, particularly those that are synthesized and secreted by the anterior pituitary gland can play important roles in a variety of physiological functions, including, for example, metabolism, temperature regulation, growth and reproduction. This family of evolutionarily conserved hormones includes the follicle-stimulating hormone (FSH), luteinizing hormone (LH), thyroid stimulating hormone (TSH), and chorionic gonadotropin (CG). Structurally, the glycoprotein hormones are heterodimers comprised of a common α-subunit and a hormone-specific β-subunit.
The two subunits are non-covalently linked to form a heterodimer, and the formation of the heterodimer has been shown to be required for receptor binding. Within a particular species, the α-subunits are identical among the glycoprotein hormones while the β-subunits differ and determine the receptor binding specificity of the particular hormone.
Structure-function relationships among the human glycoprotein hormones have been substantially based on models of gonadotropins, particularly hCG. The crystal structure of partially deglycosylated hCG revealed structural features that are also relevant to the other glycoprotein hormones (Lapthom et al., 1994, Nature 369:455-461; Wu et al., 1994, Structure 2:548-558). The common α-subunit contains an apoprotein core of 92 amino acids including 10 cysteine residues, which forms pairs by disulfide linkage. The proposed cysteine pairs are 7-31, 10-60, 28-82, 32-84, and 59-87. Bonds 28-82 and 32-84 form a ring structure penetrated by a bond bridging cysteine residues 10 and 60 to result in a core—the cystine knot—that forms three hairpin loops. Both the α-subunit and the hCG β-subunit have a similar overall topology. Each subunit has two β-hairpin loops (L1 and L3) on one side of the central cystine knot (formed by three disulfide bonds), and a long loop (L2) on the other side.
The α-subunit is encoded by a single gene which is located on chromosome 6 in humans, and is identical in its amino acid sequence within a given species (Fiddes and Goodman, 1981, J. Mol. Appl. Gen. 1:3-18). The hormone specific β-subunit genes differ in length, structural organization and chromosomal localization (Shupnik et al., 1989, Endocr. Rev. 10:459-475).
The carbohydrate moiety of the glycoprotein hormones constitutes 15-35% by weight of the hormone. The common α-subunit comprises two asparagine (N)-linked oligosaccharides, and the β-subunit has one asparagine glycosylation site in TSH and LH and two in CG and FSH. In addition, the CG β-subunit has a unique 32-residue carboxyl-terminal extension peptide (CTEP) with four serine (O)-linked glycosylation sites. (Baenziger, 1994, in: Lustbander et al. (Eds.) Glycoprotein Hormones: Structure, Function and Clinical Implications. Springer-Verlag, New York, pages 167-174).
Following secretion, the glycoprotein hormones travel in the blood stream to the target cells, which contain membrane bound receptors. The hormone binds to the corresponding receptor and stimulates the cell. Typically, such stimulation involves an increase in activity of a specific intracellular regulatory enzyme which in turn catalyzes a biochemical reaction essential to the response of the cell. For example, binding of hCG to the hCG receptor present upon the corpus luteum (an ovarian structure), stimulates the activity of the enzyme adenylate cyclase. This enzyme catalyzes the conversion of intracellular ATP to cyclic AMP (cAMP). cAMP stimulates the activity of other enzymes involved in the production of ovarian steroid hormones, especially progesterone. hCG-stimulated progesterone secretion is essential for the maintenance of pregnancy during the first trimester of gestation. The exact mechanism by which a dimeric glycoprotein hormone, such as hCG, stimulates post-receptor events, such as activation of adenylate cyclase activity, is unknown. By a variety of experimental manipulations, it has been shown however, that accurate glycosylation plays important role in this regard. Treatment of glycoprotein hormones such as LH, FSH, or hCG with hydrogen fluoride removes approximately 70% of the oligosaccharide side chains. The resultant partially deglycosylated molecules retain their receptor binding activity but are unable to stimulate any post-receptor events. Glycosylation is also important in determining the hormone plasma half-life.
Soluble proteins containing cystine knot domains such as the glycoproteins are known to bind G-protein coupled receptors; other cystine knot proteins; glycoprotein hormone superfamily members; and glycoprotein hormone receptors. Thus, these multifunctional glycoproteins modulate a number of functions, including modulation of glycoprotein hormones-related protein activity, regulation of cellular proliferation, regulation of cellular differentiation and regulation of reproductive function.
U.S. Application No. 20020169292 discloses compositions and methods based on mutant Cystine Knot Growth Factors (CKGFs) comprising amino acid substitutions relative to the previously known hormone/growth factor. Specifically the application discloses mutated thyroid stimulating hormone (TSH) and chorionic gonadotropin (CG), which possessed modified bioactivities, including superagonist activity.
U.S. Pat. No. 6,361,992 discloses modified human glycoprotein hormone comprising modified α-chain in which certain amino acids are substituted with basic amino acids, specifically modified human glycoprotein hormone having increased activity over a wild-type hormone.
U.S. Pat. No. 6,306,654 discloses recombinantly produced human FSH which offers the opportunity for control of glycosylation pattern both on the α and β portions of the heterodimer. These obtained mutants have utility as antagonists and in altering pharmacokinetic activity of these hormones.
U.S. Pat. No. 6,242,580 discloses single-chain forms of the glycoprotein hormones, which may either be glycosylated, partially glycosylated, or nonglycosylated, and the α- and β-chains that occur in the native glycoprotein hormones or variants of them may optionally be linked through a linker moiety. The resulting single-chain hormones either retain the activity of the unmodified heterodimeric form or are antagonists of this activity.
Agonist and antagonist of the glycoprotein hormone α-chain can be used for screening, diagnosis and treatment of glycoprotein hormone related conditions, diseases or disorders, and for targeting of drugs or other therapeutic entities to cells bearing glycoprotein hormone receptors.